A better understanding of the pathophysiology of Y. pestis and its interaction with the host immune system is central to the development of novel approaches for the prevention and treatment of bioterrorism associated plague. While much work has been done on the interrelationship between the bacterium and the innate immune system of the host, little is known about the mechanisms by which the bacterium is able to evade adaptive immune responses. The focus of this application will be to identify the specific effectors (Yops) used by Y. pestis to prevent adaptive immunity from developing during an infection. New experimental systems will be developed which will permit the dissection in vitro and in vivo of the effects of Yops on the adaptive immune response. The interactions of the phosphotyrosine phosphatase, YopH, with the host adaptive response will be analyzed as a prototype of this approach. Four aims will be pursued in this program. In Specific Aim 1, B cell, T cell and macrophage lines expressing Yop H will be generated and analyzed for their ability to mediate antigen dependent and independent activation and effector responses. Specific Aim 2 will further define the specific signaling pathways perturbed by Yop H expression in B cells by using DT40 clones deficient in specific signaling components that also express Yop H. These in vitro studies will provide the rationale for generating transgenic mice in Specific Aim 3 in which the expression of Yop H is targeted to specific lymphoid and myeloid compartments. These animals will be evaluated in Specific Aim 4 for their ability to develop a mature immune system and for their ability to respond to thymic dependent and independent antigens, through appropriate antigen presentation, T cell activation and B cell stimulation, affinity maturation and expansion. This experimental approach will then permit, in the future, for similar studies on other Yops and for the infection of specific Yop expressing transgenics with Y. pestis mutants defective in virulence. This approach will thus permit the evaluation of the interaction of the pathogen with adaptive immune pathways by bypassing the effects on innate responses.